At the experimental plot of Weissfluhjoch in Switzerland, 2,540 m a.s.l., tritium, deuterium and oxygen-18 have been used to study exchange processes during the build-up and ablation of the snow cover as well as the hydrological balance. Selected results from the period 1971-1979 are presented. The varying isotope concentration of the respective snow layers reveal a mass exchange between the percolating melt water and the snowpack, influencing the isotope composition of the outflow from a snow lysimeter. A correlation between the isotope concentrations in the snow cover and meltwater has been established. The enrichment of meltwater in stable isotopes enables the evaporation to be calculated independently from the hydrological measurements. Parallel tritium data point to a surprisingly high exchange between the snow surface and air moisture.